Add contribution guidelines.

This is the initial version of a contribution guidelines document,
it provides brief intructions aimed to establish unified contribution
process.

Author: Andrei Pashkin

CONTRIBUTING.rst

@@ -0,0 +1,39 @@
+=======================
+Contribution guidelines
+=======================
+These guidelines are aimed to ease code review process and establish unified
+contribution process.
+
+Submitting changes
+------------------
+- Fork and clone the repository.
+- Make a branch off of `master` branch.
+- When ready - push the branch and create a pull request to `master` branch.
+- Make sure that the CI build passes.
+
+Making commits
+--------------
+- Split your logically separate changes into separate commits.
+- Split formatting changes into separate commits.
+- Provide detailed descriptions for your commits so it would be easier for
+  the other contributors to review/investigate the code written by you.
+- The commit messages must be formatted in Linux-style way:
+    * Summary line no longer than 50-75 characters.
+    * Blank line.
+    * Explanation body wrapped at 75 columns.
+
+Code review
+-----------
+- At least 2 developers from the core team must approve a pull request before
+  it could be merged.
+
+Branching strategy
+------------------
+- The main branch is `master`.
+- Contributors create topic branches for each bugfix/feature/etc off of `master`
+  and merge them back to `master`.
+- At a release time - a release branch is made.
+- To backport a change to a release-branch - it first must be committed to
+  `master` branch or a relevant release-branch and then cherry-picked to
+  older release-branches.
+- Each release must be tagged.